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High AC current inductors

There are several issues at work here. For high AC current inductors, you want to have low core losses, low proximity loss in the windings, and low fringing effects.

At normal frequencies, ferrites are by far the lowest core loss, much better than MPP and other so called "low-loss" materials. So you would like to use them from this aspect.

A toroid gives the greatest winding surface for the magnetic material, letting you use the least number of layers and hence minimizing proximity loss. The toroid also has the advantage of putting all the windings on the outside of the structure, facilitating cooling. This is very important.

However, you can't easily gap a toroid of ferrite, it's very expensive.

Some aerospace applications actually cut the ferrite toroid into segments and reassemble them with several gaps to solve the problem. The multiple gaps keep fringing effects low. It might be nice if you could buy a set of toroidal segments so you don't have to do the cutting because that is a big part of the cost. I don't know if that is a reasonable thing to do, maybe someone can comment.

Once you go to MPP, the core loss goes up, but the distributed gap minimizes the fringing losses.

The MPP lets you run somewhat higher on current before saturation, but if you have high ac you can't take full advantage of that due to the core losses.

All these tradeoffs (and quite a few more not mentioned for brevity) are the reason that so many different solutions exist.

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